disable seqscan

Hi,
I have build an index. When, i execute the query, it gives the result by
sequential scan, not by using my index.
I have already run vacuum analyze to collect some statistics regarding
table.

May be sequential scan is giving faster execution time than my indexing. But
i want to know how much time it would take in my indexing.
For that, i have set enable_seqscan=off in postgresql.conf. But it still
going through sequential scan. Even i tried to set for a particular session,
by "set enable_seqscan=off" on psql terminal.
It again going by sequential scan.

Does any one having an idea to force postgres to use index scan?

Thanks
Nick